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String theoretic QCD axions in the light of PLANCK and BICEP2 (QCD axion after BICEP2) Kiwoon Choi @ KIAS, May 1, 2014 KC, K.S. Jeong and M.S. Seo, arXiv:1404.3880 The IBS Center for Theoretical Physics of the Universe

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Outline * Introduction * String theoretic axions * Cosmological constraints on the QCD axion * Implications

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Axion search experiments: Conversion of solar axions to X-ray photons Reasonant (CERN Axion Solar Telescope) conversion of DM axions to microwave photons: (ADMX, CAPP) The new IBS Center for Axion and Precision Physics (CAPP) @ KAIST has been launched recently for axion DM search experiment.

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ADMX (Axion Dark Matter eXperiment) vs. CAPP (Center for Axions and Precision Physics) Current plan B-field High-Q B-field

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Questions about the QCD axion: * What is the origin of the global U(1) PQ which is explicitly broken dominantly by the QCD anomaly ? U(1) PQ should be protected well from quantum gravity effects which generically do not respect global symmetries: Axion potential from QCD instantons: Axion potential from quantum gravity (stringy instantons, wormholes, …): We need an explanation for why the PQ symmetry is so well protected from quantum gravity.

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* What is the mechanism to generate the axion scale in a way consistent with the astrophysical and cosmological constraints ? Star cooling: Relic axion abundance, Axion isocurvature perturbations, … (before BICEP2) Upper bound on the axion scale depends on a variety of cosmological factors such as - Axion misalignment and quantum fluctuations during the inflation period (if the PQ symmetry were spontaneously broken during inflation, and never restored thereafter) - Dynamics of the axionic string-wall system (if the PQ symmetry were broken after the inflation is over )

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String theoretic QCD axions String theory is the right place to ask the origin of the global PQ symmetry which is well protected from quantum gravity, as it is supposed to be a theory of quantum gravity. Indeed 4D effective theory of string compactification generically involves axion-like fields originating from higher-dim antisymmetric tensor gauge fields. Witten ‘84 (Gravity-)Gauge-axion unification (extended object) (extra dimension) 4-dim global PQ symmetry which is locally equivalent to a higher-dim gauge symmetry This can explain why the PQ symmetry is so well protected from quantum gravity.

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Example: Antisymmetric tensor gauge field on 2-sphere S 2 with radius R in the internal space, with a gauge symmetry: for globally well defined Harmonic area 2-form on S 2 : Axion-like fluctuation: Global shift symmetry locally equivalent to the gauge symmetry :

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This means that U(1) shift : a st a st + constant can be broken only by non-local (in extra dim) effects associated with * Stringy instantons wrapping S 2 : Instanton amplitudes * Axion couplings to the 4-dim gauge field instantons It is then conceivable that U(1) shift is broken dominantly by the axion coupling to the QCD instantons, and the huge suppression of explicit PQ breaking by quantum gravity can be explained by * U(1) PQ = U(1) shift is locally equivalent to a higher-dim gauge symmetry. * R is moderately larger than the string length scale:

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As we will see, this simple scheme of gauge-axion unification appears to be excluded by the PLANCK constraints on isocurvature density perturbations and the recent detection of tensor modes in the CMB by BICEP2. PQ symmetry is non-linearly realized at the string scale (there is no phase of restored PQ symmetry), which results in the axion scale around the GUT scale: KC & Kim ‘85 This simple estimate of the axion scale applies for the most of compactified string models in which the compactification scale is close to the reduced Planck scale M Pl = 2.4 x 10 18 GeV. Svrcek & Witten ‘06 (cf: BICEP2 )

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Generalization with anomalous U(1) A gauge symmetry: In fact, string theory allows an interesting generalization of this elegant gauge-axion unification scheme, in which U(1) PQ is still locally equivalent to a gauge symmetry, while the axion scale can be far below the GUT scale. * Global shift symmetry which is locally equivalent to higher-dim gauge symmetry: * Anomalous U(1) gauge symmetry under which the QCD axion is charged: U(1) PQ = Combination of U(1) shift and U(1) A QCD axion = Combination of a st and arg(ф)

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Cosmological constraints on the QCD axions * Case that the PQ symmetry is spontaneoulsy broken (non-linearly realized) over the inflation period, and never restored thereafter: No axionic strings or domain walls, but the axion field could have a classical misalignment, as well as a fluctuation which originates from quantum fluctuations during the inflation period: a/f a (t 0 ) ( )

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Axion dark matter produced by the coherent oscillation of misaligned axion field:

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Axion isocurvature perturbation: Adiabatic perturbations: Isocurvature perturbations: * Inflaton (= curvature) perturbation = Adiabatic perturbation Inflation: Spontaneous breaking of the time translation invariance by a rolling inflaton field (Inflaton field = a local clock) * Axion perturbation = Isocurvature perturbation Power spectrum of the axion isocurvature perturbation:

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Before BICEP2:

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Implications for string theoretic QCD axion * String theoretic QCD axion which originates from antisymmetric tensor gauge fields with is now excluded.

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* Cosmological constraints from PLANCK and BICEP2 require that the PQ symmetry is either restored ( ), or broken at a much higher scale ( ), during inflation. This suggests that the axion scale might be generated by SUSY-breaking effects, which can be successfully implemented in string theoretic axion models with anomalous U(1) gauge symmetry: Present universe: Inflation epoch:

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More careful analysis shows that the Hubble-induced U(1) A D-term plays an important role for the symmetry breaking during inflation. An effective theory analysis: U(1) A sector: Inflaton + SUSY breaking sector: Couplings between the two sectors:

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Vacuum configuration in the present universe: ( ) Vacuum configuration during the inflation period:

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For generic parameter region, the PQ symmetry is spontaneously broken during the inflation period with although there exists an unnatural parameter range in which the PQ symmetry is restored during the inflation period.

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Most plausible range of f a (t 0 ) and Ω a in view of our results

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Conclusion * Compactified string models involving an anomalous U(1) gauge symmetry with vanishing FI-term appears to be the best theoretical setup for the QCD axion, which explains the origin of the PQ symmetry, while giving an axion scale well below the GUT scale. * PLANCK and BICEP2 results require that either the PQ symmetry is restored, or broken at much higher scale, during the inflation period. For high scale inflation ( ) suggested by the BICEP2 results, the allowed range of the axion scale and the relic axion abundance is greatly reduced. The results fit well to the scenario that the PQ symmetry is spontaneously broken by SUSY-breaking effects, leading to a specific connection between the axion scale and the SUSY breaking mass as

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* More careful analysis of the dynamics of the model implies that the PQ symmetry is spontaneously broken during inflation for generic model parameters, and the QCD axions might be here. Axion scale SUSY (n=0): or Low scale SUSY (n=1):

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